Basic principles of Java I

WYH grass sample 2021-01-21 17:42:49
basic principles java


With Java The development of ,SUN to Java There are three different versions :

Java SE:Standard Edition

  • Java EE:Enterprise Edition
  • Java ME:Micro Edition
  • What is the relationship between the three ?

┌───────────────────────────┐
│Java EE │
│ ┌────────────────────┐ │
│ │Java SE │ │
│ │ ┌─────────────┐ │ │
│ │ │ Java ME │ │ │
│ │ └─────────────┘ │ │
│ └────────────────────┘ │
└───────────────────────────┘
Simply speaking ,Java SE It's the standard version , Include standard JVM And the standard library , and Java EE It's the enterprise edition , It's just Java SE And a lot of API And Ku , In order to facilitate the development of Web application 、 database 、 Message service, etc ,Java EE The application uses virtual machines and Java SE Exactly the same .
Java ME Just like Java SE Different , It's an embedded device “ Slimming version ”,Java SE The standard library of can't be in Java ME Upper use ,Java ME The virtual machine is also “ Slimming version ”.
Beginner study Java, Often hear JDK、JRE These nouns , What are they ?

  • JDK:Java Development Kit
  • JRE:Java Runtime Environment
    In short ,JRE Is to run Java The virtual machine of bytecode . however , If only Java Source code , To compile into Java Bytecode , Need JDK, because JDK In addition to containing JRE, There's also a compiler 、 Debugger and other development tools .

The relationship between them is as follows :

┌─ ┌──────────────────────────────────┐
│ │ Compiler, debugger, etc. │
│ └──────────────────────────────────┘
JDK ┌─ ┌──────────────────────────────────┐
│ │ │ │
│ JRE │ JVM + Runtime Library │
│ │ │ │
└─ └─ └──────────────────────────────────┘
┌───────┐┌───────┐┌───────┐┌───────┐
│Windows││ Linux ││ macOS ││others │
└───────┘└───────┘└───────┘└───────┘
To learn Java Development , Of course it needs to be installed JDK 了 .

install JDK Set the environment variable
JDK

stay JAVA_HOME Of bin Many executable files found in the directory :

  • java: This executable program is actually JVM, function Java Program , That is to start. JVM, And then let JVM Execute the specified compiled code ;
  • javac: This is a Java The compiler , It is used to put Java The source code file ( With .java The suffix ends ) Compiled into Java Bytecode file ( With .class The suffix ends );
  • jar: Used to put a group of .class The files are packed into one .jar file , Easy to release ;
  • javadoc: For from Java Automatically extract comments from source code and generate documents ;
  • jdb:Java The debugger , It is used for operation and debugging in the development stage .

install Java

  1. JDK You can download it on the official website


Click next . Select installation path .

After waiting , Select installation location .

Click next , Continue to wait for .

installation is complete .

Set the environment variable

Right click on the desktop My computer → attribute → Advanced system setup → Advanced options → environment variable
Click New in the system variable on the lower side


Enter variable name :JAVA_HOME, Variable value is jdk Installation position of .

Click OK , New variable again .
A variable called :CLASSPATH, Variable value is :.;%JAVA_HOME%\lib\dt.jar;%JAVA_HOME%\lib\tools.jar;

Click OK , find PATH Variable , Click edit . Add the following two variable values .

Finally, click OK . After all are confirmed .
function cmd, perform java -version. The following information is displayed . Proof of successful installation .

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